1. Field
This invention relates to an improvement in the preparation of the sorbent beds of a simulated moving bed chromatographic separator. It is particularly directed to a sorbent bed packing technique which reduces void volume and improves the separation characteristics of a simulated moving bed.
2. State of the Art
Batch, continuous or simulated moving bed systems designed for the chromatographic separation of mixture components often consist of one or more beds of solid separator medium (sorbent). Proper preparation of the sorbent bed(s) is an important factor, and determines in part the separating efficiency of the chromatographic device.
It is generally understood that a sorbent bed should be prepared in such a manner that no large voids exist in the compartment containing the sorbent. This preparation minimizes mixing inside the compartment and improves even plug flow across the compartment cross-section. It is also recognized that an increase in sorbent bed density can improve the separating efficiency of the sorbent bed.
Preparation procedures which have been practiced include:
(1) "Slurry" or "dry" packing, in which the sorbent is pumped or drawn into the bed compartment at atmospheric pressure.
(2) "Slurry" packing, in which sorbent is mixed with a solvent and the slurry is pumped into the compartment under pressures greater than that to be applied in the subsequent separation procedure.
(3) "Slurry" or "dry" sorbent packing followed by vibration of the sorbent compartment more densely to pack the sorbent.
(4) Axial or radial compression systems which mechanically pack the sorbent.
(5) Any of the procedures (1), (2), (3) or (4), followed by contacting the bed with flowing liquid at high pressure to compress the sorbent bed, thereby to create a void in the bed, and thereafter filling the resultant void with additional sorbent.
(6) Packing methods which involve shrinking the sorbent and then expanding it in some manner so that the sorbent is packed throughout the compartment. For example, U.S. Pat. No. 4,336,060 describes a method in which a resin is contracted with a salt solution and is then transferred in the shrunken state to a separation column. The loaded resin completely fills the column. The confined resin is then washed with water to remove unbound salt. The resin then expands, creating a positive expansion pressure within the separator column. U.S. Pat. No. 4,673,507 discloses a similar process for ion exchange type applications.
Sorbent bed systems representative of the types contemplated by this invention are disclosed in U.S. Pat. Nos. 2,985,589; 3,831,755; 4,400,278; 4,404,037; 4,011,113; 4,182,633; 4,247,636; 4,412 866; 4,501,814 and 4,511,476, the disclosures of which are incorporated by reference for their teachings concerning sorbent beds generally and the use of such beds in industrial scale operations. U.S. Pat. 4,511,476 is specifically instructive concerning the mechanical manipulation of such beds. The disclosures of U.S. Pat. Nos. 4,366,060 and 4,673,507 are incorporated by reference for their disclosures concerning techniques for shrinking and expanding resins useful for sorbent beds.
Simulated moving bed (SMB) technology is well developed for applications involving separating the components of a fluid. Typical applications of simulated moving bed chromatography include the separation of fructose from fructose-glucose solutions and the separation of sucrose from sugar beet or sugar cane syrups. Ion exchange resins are typically employed as sorbents for these applications. Solution components are differentially absorbed by the ion exchange resin so that a separation waveform develops within the simulated moving bed.
A typical simulated moving bed apparatus consists of several compartments (or individual columns) filled with solid sorbent. A fluid conduit interconnects the upstream and downstream ends of the system to form a loop through which fluid is continuously recirculated. The constant flow of fluid through the loop is called "internal recirculation flow." A manifold system of pipes and valves is provided selectively to position an inlet for feed material, an inlet for desorbent, an outlet for a sorbed component and an outlet for a nonsorbed (or less sorbed) component. Each inlet and outlet communicates with a separate bed compartment. Feed material enters the system at a designated compartment and is moved through the sorbent by the continuous internal recirculation flow. This moving contact results in a chromatographic separation of components. Sorbed component(s) which flow(s) at a relatively slow rate is removed from the sorbed component outlet. Nonsorbed component(s) which flow(s) at a relatively fast rate is removed from the nonsorbed component outlet. Desorbent is added at its inlet valve between the respective outlet valve positions of the sorbed and nonsorbed components.
At predetermined time intervals (step time) the designated inlet and outlet valve positions are displaced downstream one position on the manifold to the next sorbent bed compartment, which may be a discrete section of a vessel, (such as a column), or an individual such vessel, e.g., column. The step time is chosen such that the designation of valves is properly synchronized with the internal recirculation flow. Under these conditions the system eventually reaches a steady state with specific product characteristics appearing at predetermined intervals in sequence at each valve position. This type of system simulates valves held in a single position while the solid sorbent moves at a constant and continuous rate around the recirculation loop producing constant quality product at each valve.
The simulated version more closely approaches the character of an actual moving bed system as the number of compartments and valve positions increase. An important distinction between batch and simulated moving bed systems is that the internal recirculation flow is continuous in the simulated moving bed process. Except for very small adjustments to control internal pressure, the entering and exiting flow rates are continuous and constant, thereby approximating an actual moving bed system as closely as possible.